Turbidimeters are devices used to detect and measure light scattered by turbid media when a beam of light is directed through the media. A conventional turbidimeter includes a light source, a sample cell, and an optical detector at a predetermined position and orientation (usually 90.degree.) with respect to the light source and the sample being tested. The optical detector produces a relatively low-level analog signal in response to the presence of turbid media, with the signal being proportional to the turbidity of the sample. The signal generated by the detector is amplified by low-bandwidth electronics to levels consistent with output electronics, chart recorders, etc.
A typical use for turbidimeters is in measuring the clarity of drinking water. In the United States, USEPA Method 180.1 regulates turbidimetric instruments and procedures used for quality control of drinking water. In Europe, ISO 7027 regulates turbidimetric instruments and procedures. Laboratory and on-line process control turbidimeters of the type regulated by USEPA Method 180.1 and ISO 7027 are marketed by several companies, notably by Hach Company.
USEPA Method 180.1 and ISO 7027 regulate various aspects of the optical design of turbidimeters, including the wavelength of the illuminating beam. Method 180.1 mandates an incandescent lamp with a color temperature of between 2200.degree. K. and 3000.degree. K., a peak detected wavelength between 400 and 600 nm, and an unspecified optical bandwidth. On the other hand, ISO 7027 specifies a peak wavelength of 860 nm and a bandwidth of 60 nm, but does not specify the particular type of light source which may be used. This has resulted in the recent introduction of ISO 7027-compliant turbidimeters which comprise solid-state light sources, such as IRLEDs (infrared light-emitting diodes).
IRLEDs are much more robust and reliable than incandescent lamps. In addition, IRLEDs are small and consume little electrical power. Such advantages are commensurate with the design of hand-held battery-operated turbidimetric instruments. Unfortunately, the available optical power in a collimated beam from a typical IRLED will generally be lower than is available in a collimated beam from an incandescent lamp. Therefore, the signal processing circuitry in an IRLED-based instrument intended for use in making low-level turbidimetric measurements (e.g., for drinking water) per ISO 7027 will require higher levels of signal amplification, as compared with the signal processing circuitry in a Method 180.1 turbidimeter. The increased level of signal amplification required in an IRLED-based instrument may result in increased susceptibility to stray light and EMI, with possible adverse effects on the dynamic range.
In prior art turbidimeters, susceptibility to stray light is reduced by placing apertures in the illuminating beam at locations which serve to confine the illuminating beam to a geometrically well-defined area. The detector is then placed outside the path of the main beam. The detector may be placed behind additional apertures to help eliminate secondary stray light. Usually, however, some stray light will find its way to the detector by means of multiple scattering paths.
The desired effect of the apertures is to eliminate sources of stray light which appear within the field of view of the detector. Ideally, the field of view of the detector should be quite large, in order to efficiently collect the turbidimetric signal which originates in a "cloud of light" generated by the presence of the illuminating beam within the sample. At the same time, the effective optical collection area of the detector should also be large. The product of the effective optical collection area of the detector times the solid angle representing the field of view is a measurement of the optical efficiency or throughput, sometimes called the etendue.
It is desirable to maximize the etendue in order to reduce the required level of signal amplification, but only if the majority of stray light remains outside the field of view of the detector. In a properly-designed turbidimeter, most stray light originates at or beyond the locations where the illuminating beam enters and exits the sample cell or compartment. Therefore, as viewed from the location of the detector, there is (or should be) a relatively well-defined angular separation between the field of view comprising the turbidimetric signal and regions outside this field which contain the stray light component. Thus, it is desirable to implement means for achieving a sharp cutoff in the field of view of the detector. It is apparent that the unapertured detectors commonly used in prior art turbidimeters do not have the required sharp field of view cutoff characteristics. The use of one or more apertures in front of the detector, also common practice in the prior art, compromises etendue and reduces the sensitivity of the instrument.
It is evident that stray light-limiting apertures work relatively well in conditions where there are large spaces and long optical paths, such as in prior art process control or laboratory turbidimeters. However, in a hand-held instrument, the distance from the collimating lens to the location of the sample, and from the sample cell to the detector, is quite limited, with insufficient room for effective use of apertures.
Thus, the optical design of prior art turbidimeters does not adequately address the need for simultaneous large field of view, large effective optical collection area, and sharp angular (field of view) cutoff. These features are especially important for compact hand-held IRLED-based instruments, where the optical power in the illuminating beam is somewhat less than has been available with the use of incandescent lamps, and where there is insufficient room for effective use of apertures to reduce the level of stray light.